Summary:
**Context/Summary:**
https://github.com/facebook/rocksdb/pull/11424 made me realize there are a couple gaps in my `ExpectedValue` comments so I updated them, along with separating `ExpectedValue` into separate files so it's clearer that `ExpectedValue` can be used without updating `ExpectedState` (e.g, TestMultiGet() where we care about value base of expected value but not updating the ExpectedState).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11456
Test Plan: CI
Reviewed By: jowlyzhang
Differential Revision: D45965070
Pulled By: hx235
fbshipit-source-id: dcee690c13b00a3119757ea9d43b646f9644e1a9
